This invention relates to an ultrasonic Doppler blood flow meter, and more particularly to a blood flow meter of the type described above in which the manner of measurement of the frequency of echoes of ultrasonic wave reflected from a living body is improved.
Prior art, ultrasonic Doppler blood flow meters are disclosed in, for example, C. Kasai et al, "Real-Time Two-Dimensional Blood Flow Imaging Using an Autocorrelation Technique" IEEE Transactions on Sonics and Ultrasonics, Vol. SU-32, No. 3, May 1985, pp. 458-464 and D. W. Baker, "Pulsed Ultrasonic Doppler Blood-Flow Sensing" IEEE Transactions on Sonics and Ultrasonics, Vol. SU-17, No. 3, July 1970, pp. 170-185. Each of the disclosed devices is essentially composed of a driver transmitting an ultrasonic wave signal toward and into a living body, a receiver receiving an echo signal of the transmitted ultrasonic wave signal, an oscillator generating an oscillation output signal having a pulse repetition frequency n times (n: an integer) as high as the repetition frequency of ultrasonic wave transmission, and a signal processing circuit processing the received echo signal. The method employed in the disclosed devices comprises transmitting the ultrasonic wave signal at the predetermined period toward a blood vessel in a living body, receiving an echo signal of the transmitted ultrasonic wave signal reflected by blood flow in the blood vessel, and measuring the Doppler shift frequency of the echo signal to measure the velocity and direction of blood flow in the blood vessel. By the above mentioned, the value of v.multidot.cos .beta. can be measured, where .beta. is the angle defined between the direction of blood flow and the direction of transmission of the ultrasonic wave signal, and v is the velocity of blood flow.
For the purpose of blood flow measurement in the manner described above, methods such as a zero-cross method and a fast Furrier transform (FET) method are commonly used. However, the latter method requires many hardware parts. with a view to decrease the number of hardware parts while taking into consideration the factors such as the accuracy of blood flow measurement, devices similar to a blood flow measuring device as shown in FIG. 1 are now proposed, as disclosed in, for example, JP-A-58-188433, JP-A-62-41645, JP-A-60-119929 and JP-A-61-25527. Referring to FIG. 1, a received input signal 7a is applied to quadrature detectors 11a and 11b. In the detector 11a, the input signal 7a is multiplied by a cosine wave signal 3b (described later) to provide an analog output signal 30a representing a real component (R=.alpha. cos .theta.), while in the detector 11b, the input signal 7a is multiplied by a sine wave signal 3a (described later) to provide an analog output signal 31a representing an imaginary component (I=.alpha. sin .theta.). These analog signals 30a and 31a are then A/D converted by A/D converters 12a and 12b respectively, and these digital signals representing the R and I components respectively are used to measure the Doppler shift phase angle thereby displaying the velocity v of blood flow on a display unit 20.
In the blood flow measuring device shown in FIG. 1, an oscillator 2 generates a stable high-frequency oscillation output signal which is applied to a frequency-dividing and synchronizing circuit 3. In response to the application of the high frequency signal, the circuit 3 generates a digital pulse signal 3d for ultrasonic pulse beam transmission, a sine wave signal 3a and a cosine wave signal 3b for quadrature detection, and a reset pulse signal 3c having a period n times (n: an integer) as large as that of the pulse signal 3d.
In response to the application of the digital pulse signal 3d, a driver circuit 4 applies an analog pulse signal 4a having, for example, a 1/2 cycle pulse to a probe 6 through a transmit/receive change-over circuit 5. The probe 6 is excited to transmit an ultrasonic pulse beam toward a blood vessel 9 of a living body 10 to be examined.
The signal reflected from the blood vessel 9 of the living body 10 is converted by the probe 6 into an electrical signal, and this electrical signal is applied through the transmit/receive change-over circuit 5 to a high frequency amplifier 7 to be amplified and appears as a receive input signal 7a which is applied to the quadrature detectors 11a and 11b. FIG. 2 shows waveforms of the signals 3a, 3b, 3c, 3d, 4a and 7a shown in FIG. 1. The received input signal 7a having a waveform as shown in (f) of FIG. 2 is applied to the qudrature detectors 11a and 11b which are in the form of multipliers. In the detectors 11a and 11b, the receive input signal 7a is multiplied by the cosine and sine wave signals 3b and 3a having waveforms as shown in (c) and (b) of FIG. 2 to appear as the analog output signals 30a and 31a representing the R and I components respectively.
These analog signals 30a and 31a are then converted into digital signals 30b and 31b by the A/D converters 12a and 12b respectively, and these digital signals 30b and 31b are passed through MTI (moving target indication) filters 13a and 13b to appear as signals 30c and 31c respectively. These signals 30c and 31c are then applied to an amplitude and phase angle calculator 14, and an output signal 32 representing the amplitude .alpha..sub.i and an output signal 34 representing the phase angle .theta..sub.i are generated from the calculator 14. On the basis of the phase angle .theta..sub.i shown in (f) of FIG. 2, the Doppler shift phase angle .DELTA..theta..sub.i (.DELTA..theta..sub.i =.theta..sub.i -.theta..sub.i-1) is calculated, and, in order to improve the S/N ratio, the mean value .DELTA..theta. of a plurality of such Doppler shift phase angles is calculated by a mean Doppler shift phase-angle calculator 18. That is, the signal 34 representing the phase angle .theta..sub.i is applied, on one hand, directly and, on the other hand, through a delay element 16, to a subtractor 17. The delay element 16 has a delay time corresponding to one period T of the pulse signal 4a. Therefore, the subtactor 17 generates an output signal 35 representing the difference .DELTA..theta..sub.i (.DELTA..theta..sub.i =.theta..sub.i -.theta..sub.i-1) between the present phase angle .theta..sub.i and the preceding phase angle .theta..sub.i-1, and such a signal 35 is applied to the mean Doppler shift phase-angle calculator 18. The calculator 18 calculates the mean value .DELTA..theta.(.DELTA..theta.=(.SIGMA.(.theta..sub.i -.theta..sub.i-1)/n) of n consecutive Doppler shift phase angles .DELTA..theta..sub.i, and its output signal 38 representing .DELTA..theta. is applied to the display 20. Thus, when the value of n is, for example, four, the reset pulse signal 3c having a waveform as shown in (e) of FIG. 2 has a period which is five times as large as that of the pulse signal 4a having a waveform as shown in (d) of FIG. 2. On the other hand, the signal 32 representing the amplitude .alpha..sub.i is applied to a mean amplitude calculator 15. The calculator 15 calculates the mean value .alpha.(.alpha.=.SIGMA..alpha..sub.i /n) of n consecutive amplitude values .alpha..sub.i, and its output signal 33 representing .alpha. is applied to the display 20.
In the display 20, the mean value of Doppler shift phase angle .DELTA..theta. or the mean value of amplitude .alpha. is displayed independently. The former relates the velocity, so does the latter the power of blood flow.
The prior art, blood flow measuring device shown in FIG. 1 has had such a problem that, in the calculation of the mean Doppler shift frequency, that is, the mean Doppler shift phase angle, the direction of the mean Doppler shift phase angle is not always the same as the direction of the Doppler shift phase angle as shown in FIG. 3A. This is because the Doppler shift phase angle that can be displayed is limited to within the range of -.pi. to +.pi. due to the structural limitation of the circuit. Therefore, the mean Doppler shift phase angle cannot be determined by merely simply calculating the numerical values of the Doppler shift phase angles. When, for example, the input signal 35 that is sequentially applied to the mean Doppler shift phase angle calculator 18, is successively representative of 170.degree., 175+, -175.degree. and -170.degree. as shown by the arrows a, b, c and d in FIG. 3A, mere addition of these numerical values and division of the sum by the factor of four does not provide a mean Doppler shift phase angle of 180.degree. as shown by the arrow e in FIG. 3A, but provides a mean Doppler phase shift angle of 0.degree. as shown by the arrow e' in FIG. 3A. FIG. 3B illustrates that the mean value shown by e' is 0 degrees.
The above problem is attributable to the fact that the displayable range of the Doppler shift phase angle is limited so as to simplify the hardware design for the purpose of minimizing the scale of the circuit.